1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of reporting CQI and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmission/reception, UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
In certain situations, the UE needs to obtain (e.g., measure) channel state information (CSI) and to report the CSI to the network. In general, the UE transmits a channel quality indicator (CQI) in a CQI mapping table (e.g., defined in the 3GPP standard) corresponding to the CSI to report the CSI. However, the CSI may be too poor such that a proper CQI corresponding to the CSI cannot be found in the CQI mapping table. In this situation, the UE may transmit the lowest CQI (i.e., the lowest CQI level) to the network. The network simply knows that the UE is with a poor channel without knowing how poor the channel is, after receiving the lowest CQI. Accordingly, the network may determine not to communicate with the UE, or may communicate with the UE according to an improper modulation and coding scheme (MCS) determined according to the received CQI. As a result, throughput of UE may be degraded.
Thus, how to report the CSI accurately is an important problem to be solved.